1. Field of the Invention
The present invention relates to a compression ignition type engine.
2. Description of the Related Art
In the past, in an internal combustion engine, for example, a diesel engine, the production of NOx has been suppressed by connecting the engine exhaust passage and the engine intake passage by an exhaust gas recirculation (EGR) passage so as to cause the exhaust gas, that is, the EGR gas, to recirculate in the engine intake passage through the EGR passage. In this case, the EGR gas has a relatively high specific heat and therefore can absorb a large amount of heat, so the larger the amount of EGR gas, that is, the higher the EGR rate (amount of EGR gas/(amount of EGR gas+amount of intake air), the lower the combustion temperature in the engine intake passage. When the combustion temperature falls, the amount of NOx produced falls and therefore the higher the EGR rate, the lower the amount of NOx produced.
In this way, in the past, the higher the EGR rate, the lower the amount of NOx produced can become. If the EGR rate is increased, however, the amount of soot produced, that is, the smoke, starts to sharply rise when the EGR rate passes a certain limit. In this point, in the past, it was believed that if the EGR rate was increased, the smoke would increase without limit. Therefore, it was believed that the EGR rate at which smoke starts to rise sharply was the maximum allowable limit of the EGR rate.
Therefore, in the past, the EGR rate was set within a range not exceeding the maximum allowable limit (for example, see Japanese Unexamined Patent Publication (Kokai) No. 4-334750). The maximum allowable limit of the EGR rate differed considerably according to the type of the engine and the fuel, but was from 30 percent to 50 percent or so. Accordingly, in conventional diesel engines, the EGR rate was suppressed to 30 percent to 50 percent at a maximum.
Since it was believed in the past that there was a maximum allowable limit to the EGR rate, in the past the EGR rate had been set so that the amount of NOx and smoke produced would become as small as possible within a range not exceeding that maximum allowable limit. Even if the EGR rate is set in this way so that the amount of NOx and smoke produced becomes as small as possible, however, there are limits to the reduction of the amount of production of NOx and smoke. In practice, therefore, a considerable amount of NO and smoke continues being produced.
The present inventors, however, discovered in the process of studies on the combustion in diesel engines that if the EGR rate is made larger than the maximum allowable limit, the smoke sharply increases as explained above, but there is a peak to the amount of the smoke produced and once this peak is passed, if the EGR rate is made further larger, the smoke starts to sharply decrease and that if the EGR rate is made at least 70 percent during engine idling or if the EGR gas is force cooled and the EGR rate is made at least 55 percent or so, the smoke will almost completely disappear, that is, almost no soot will be produced. Further, they found that the amount of NOx produced at this time was extremely small. They engaged in further studies later based on this discovery to determine the reasons why soot was not produced and as a result constructed a new system of combustion able to simultaneously reduce the soot and NOx more than ever before. This new system of combustion will be explained in detail later, but briefly it is based on the idea of stopping the growth of hydrocarbons into soot at a stage before the hydrocarbons grow to soot.
That is, what was found from repeated experiments and research was that the growth of hydrocarbons into soot stops at a stage before that happens when the temperatures of the fuel and the gas around the fuel at the time of combustion in the combustion chamber are lower than a certain temperature and the hydrocarbons grow to soot all at once when the temperatures of the fuel and the gas around the fuel become higher than a certain temperature. In this case, the temperatures of the fuel and the gas around the fuel are greatly affected by the heat absorbing action of the gas around the fuel at the time of combustion of the fuel. By adjusting the amount of heat absorbed by the gas around the fuel in accordance with the amount of heat generated at the time of combustion of the fuel, it is possible to control the temperatures of the fuel and the gas around the fuel.
Therefore, if the temperatures of the fuel and the gas around the fuel at the time of combustion in the combustion chamber are suppressed to less than the temperature at which the growth of the hydrocarbons stops midway, soot is no longer produced. The temperatures of the fuel and the gas around the fuel at the time of combustion in the combustion chamber can be suppressed to less than the temperature at which the growth of the hydrocarbons stops midway by adjusting the amount of heat absorbed by the gas around the fuel. On the other hand, the hydrocarbons stopped in growth midway before becoming soot can be easily removed by after-treatment using an oxidation catalyst etc. This is the basic thinking behind this new system of combustion.
In this new method of combustion, however, it is necessary to make the EGR rate at least more than 55 percent so as to simultaneously reduce the soot and NOx. It is however only possible to make the EGR rate more than about 55 percent when the amount of intake air is small, that is, when the engine load is relatively low. Once the amount of intake air exceeds a certain limit, it is no longer possible to increase the amount of intake air without lowering the EGR rate. When the amount of intake air exceeds a certain limit, however, that is, when the amount of fuel injection is relatively large, if the EGR is allowed to gradually fall from 55 percent along with the increase of the amount of intake air, the problem will arise of the production of a large amount of smoke.